The purpose of this study is to evaluate if the medication called CellCept or mycophenolate mofetil can be used to safely treat patients who have had a liver transplant and have abnormal kidney function as a result of taking FK506 (tacrolimus, Prograf). Mycophenolate mofetil (MMF) is a Food and Drug Administration (FDA) approved medication used to suppress the immune system in kidney transplant patients in order to prevent rejection of the new organ after transplant. MMF has been shown to NOT be toxic (causing ill effects) to kidney cells. This study will examine the effectiveness (how well it works) and safety (side effects) of taking MMF and prednisone in combination with either no dose of FK506, or with decreasing the dose of FK506 by half.